As a drill having a plurality of cutting edges, a two-blade drill including two cutting edges and two helical flutes connected to each of the cutting edges has conventionally been known. These two flutes of the two-blade drill are extended to the peripheral section of the drill in the same shape and at a certain helix angle. However, each flute contributes to a decrease in the cross-sectional area of the drill. Hence, the rigidity of the drill may be lowered when a plurality of such flutes are formed.
In contrast, for example, Japanese Unexamined Patent Publication No. 2007-307642 discloses that in the drill having, on the front end of the body thereof, two cutting edges and two helical flutes respectively connected to these two cutting edges, these two helical flutes are joined together into a single flute at a position retreated by a predetermined amount from the front end of the body.
However, in the drill having these two flutes thus joined together, the chips generated from each of the cutting edges tend to clog at the junction of the flutes. As a result, the clogged chips cause the generation of heat at the junction, and consequently there is a risk that the workpiece is deformed, or the inner wall of a drilled hole is deformed (surface roughness is deteriorated). There is also a risk that the chips clogged at the junction increases the stress exerted on the junction during drilling (cutting torque), thus causing the drill to fracture. On the other hand, these two flutes affect each other at the junction, thus causing changes in the flute shape. Consequently, there is a risk that the flow of chips passing through each of the flutes is changed, thus affecting the inner wall of the drilled hole.
Hence, there are a need for a drill having both high-quality hole drilling characteristics and sufficient rigidity, and a need for a method of cutting a workpiece using the drill.